The present invention relates generally to fault testing of insulation applied to a cable, wire or other elongated electrical conductor and deals more particularly with insulation testing apparatus of the type wherein a high test voltage is applied between the insulation and the conductor to reveal holes, voids or other imperfections in the insulation.
Insulated electrical conductors, such as wire or cable, are usually subjected to a high voltage test prior to use as a check against any unseen imperfections which may be present in the insulation. This test may be made either as the insulation is extruded onto the conductor or may be performed in some subsequent operation. Generally, the insulated conductor, which is suitably grounded, is passed through an electrode of some kind which applies a high voltage to the outer surface of the insulation. Typically, the test voltage may be an alternating voltage or a direct or unidirectional test voltage. In either case, the test voltage produces a spark or arc upon the appearance of an imperfection at the electrode. In addition, the testing apparatus generally includes a counter, alarm and/or other indicator or control which is operated in response to the appearance of such an arc.
In my U.S. Pat. No. 3,418,570 issued Dec. 24, 1968 entitled ELECTRICAL DEVICE FOR TESTING FOR AND COUNTING FLAWS IN THE INSULATION OF AN ELECTRICAL CONDUCTOR PASSING THROUGH AN ELECTRODE, apparatus is disclosed which overcomes a number of drawbacks related to the speed of movement of the conductor through the electrode, multiple flaw counts and damage to the insulation under test, some or all of which drawbacks are associated with prior art devices. Reference may be had to the above-referenced patent for a detailed description of the operation and construction of the testing device and which disclosure is incorporated herein by reference.
Despite the improvements in testing the insulation of an electrical conductor with the apparatus disclosed in the above-referenced patent, the testing apparatus exhibits a number of drawbacks which are overcome by the present invention and which are described below.
One drawback of the prior testing apparatus is the low efficiency of the oscillator circuit which tends to limit the output power, and accordingly, distort the sinusoidal output wave-form used to produce the test voltage.
Another drawback is that the frequency of the oscillator tends to shift to a high radio frequency when the oscillator and the high voltage transformer were separated by any appreciable distance result of such to the magnitude of the high voltage output drops to a low value impairing the operation of the apparatus as a flaw detector.
Another drawback associated with the prior testing apparatus is its use of power loss as a fault criterion. Since power loss could occur under a number of conditions including, an arc through a flaw; the power lost through corona discharge and power loss within the insulation of the conductor under test, false fault indications could be recorded.
A further drawback of the prior test apparatus is the resultant damage to the oscillator electronic components, the voltage regulator and the DC power supply of the apparatus when long intervals of bare wire pass through the electrode, or when the conductor under test remains stationary in the electrode for a sufficient time to destroy the insulation. Under these and other conditions, the oscillator circuit ceases to operate and excessive power is consumed.
A further drawback associated with the above-referenced prior testing device was that the oscillator output voltage is not a linear function of the applied direct voltage, particularly at low voltages. Since the relationship of the oscillator output voltage to the applied direct voltage is part of the output voltage control loop, control loop oscillations tend to occur at low output voltages. This problem is compounded since control loop gain is varied to achieve a desired output voltage control with the result that output voltages of less than 20% of full scale cannot be achieved.
It is desirable therefore to overcome the drawbacks and limitations generally associated with AC and unidirectional current testing apparatus such as the prior testing apparatus described above and to further provide improvements over the limitations in the testing apparatus disclosed in the above-referenced patent.
It is therefore a general aim of the present invention to provide an improved apparatus for testing and counting flaws in the insulation of an electrical conductor passing through an electrode by providing an oscillator circuit having a high efficiency and a very linear relationship between the applied direct voltage and AC output voltage at and including very low voltage levels.
It is also an object of the present invention to provide testing apparatus which utilizes a control loop wherein operation is possible over substantially a 150 to 1 ratio of voltages without control loop oscillation.
It is a further object of the present invention to permit the high voltage transformer associated with the apparatus to be located several hundred feet from the oscillator without introducing instability due to radio frequency parasitic oscillations.
It is a yet further object of the present invention to provide a control loop wherein constant gain is maintained as the magnitude of the output voltage is varied to limit the direct current supplied to the oscillator when a short circuit condition is sustained in the electrode.